Viscous (fluid shear) clutches have been employed in connection with the torque converter of motor vehicle drivetrains to improve drivetrain efficiency. In such applications, the viscous clutch is referred to as a viscous converter clutch or VCC. The torque converter is connected between the engine and transmission and defines a relatively high slip torque transmission path therebetween. The VCC is selectively connected in parallel with the torque converter via engagement of a friction clutch to define an alternate, relatively low slip, torque transmission path between the engine and transmission.
Engagement of the friction clutch is generally controlled in accordance with the operating conditions of the drivetrain. When the friction clutch is disengaged, engine torque is transmitted to the transmission through the torque converter. When the friction clutch is engaged, engine torque is transmitted to the transmission through the friction clutch and the VCC. The relatively low slip of the activated VCC improves drivetrain efficiency, while maintaining a smoothing effect on the transmission input torque for good drivability.
The slip across an activated VCC exhibits a characteristic variation as a function of the input (engine) speed and torque and the viscosity of the internal VCC fluid. The viscosity of the fluid varies with its temperature, which in turn, depends on the internally generated fluid shear and the ambient oil temperature surrounding the clutch.
Under certain heavy load conditions, such as when the vehicle is pulling a trailer up a long grade in high ambient temperatures, the fluid in the VCC may overheat. This is potentially harmful to the clutch since the reduced viscosity of the fluid permits increased slippage, and therefore, further heating of the fluid. In extreme cases, the fluid may become chemically degraded and clutch damage may occur.
Ideally, one might avoid damage to the clutch under the above described operating conditions by simply measuring the temperature of the clutch fluid and disengaging the friction clutch when the measured temperature exceeds a reference temperature. However, this is a rather expensive and impractical approach due to the clutch environment.